Defrost control for refrigeration apparatus



Aug. 15, W67 u s. PHILLIPS EFROST CONTROL FOR REFRIGERATION APPARATUS Filed Sept. 15, 1965 1 y no A WINK). L

Q mm; r 1 I y "r IL Iii; \LQI Q United States Patent 3,335,576 DEFRGST CGNTRGL FOR REFRIGERATION APPARATUS Donald S. Phillips, Evansville, Ind., assignor to Whirlpool Corporation, a corporation of Delaware Filed Sept. 15, 1965, Ser. No. 487,561 8 Claims. (Cl. 62-156) ABSTRACT OF THE DISCLOSURE A defroster control for use in refrigerators and the like utilizing a plurality of thermistor solid state controls including manually adjustable variable potentiometers for adjusting the defrosting cycle of the refrigerator apparatus. The controls are associated with the evaporator of the refrigerator apparatus to initiate a defrost cycle when the temperature sensed by one thermistor reaches a preselected low temperature and to discontinue the defrost operation when the temperature sensed by another thermistor reaches a preselected high temperature.

This invention relates to refrigeration apparatus and in particular to control systems for use in connection with defrosting of such refrigeration apparatus.

In conventional refrigeration apparatus such as refrigerators and freezers, it is desirable at times to effect a defrosting of the refrigerated means. Such defrosting has heretofore been effected by manual initiation by the user, or by automatic initiation as by time switch controls and the like. More recently, automatic defrost controls have been provided wherein a sensing of the need for initiating the defrosting operation is effected by means of a thermostat device disposed in thermal transfer association with the refrigerated means. None of the present control devices have proven completely satisfactory, and it is a principal object of the present invention to provide a new and improved control device for controlling the defrosting in such refrigeration apparatuses.

Thus, a principal feature of the present invention is the provision of a new and improved defrost control for use in refrigeration apparatus.

Another feature of the invention is the provision of such a defrost control having simple and economical construction while yet providing improved sensitivity and accuracy in the controlling of the defrost operation.

A further feature of the invention is the provision of such a defrost control for use in a refrigeration apparatus having refrigerated means and means for defrosting the refrigerated means, wherein the control comprises a first thermistor in thermal transfer association with the refrigerated means for sensing a requirement of defrosting thereof and having current control means to vary electrical current flow therethrough, a second thermistor in thermal transfer association with the refrigerated means for sensing a completion of a defrosting of the refriger ated means and current control means connected in series with the second thermistor to vary electrical current flow therethrough, and relay means having switch means electrically connected to the defrosting means, and coil means electrically connected to the current control means of the thermistors for operating the switch means as a function of the current flow through the current control means, the current control means being preselected to permit a sufiicient current to flow to operate the relay means to energize the defrosting means when the first thermistor senses a defrost requirement and to maintain a sufficient current flow to hold the relay means energized until the second thermistor senses a completion of a defrosting of the refrigerated means.

Still another feature of the invention is the provision 3,335,576 Patented Aug. 15, 1967 of such a defrost control wherein the relay means comprises a pair of relays, a first one of which includes a coil connected in series with the current control means and switch means, and the second one of which includes a transducer connected in series with the switch means of the first relay and switch means connected in series with the defrosting means.

Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawing which comprises a schematic diagram illustrating the electrical circuitry of the control apparatus embodying the invention.

In the exemplary embodiment of the invention as disclosed in the drawing, a defrost control generally designated 10 is shown to comprise a defrost heater 11 suitably disposed within an evaporator compartment 12 of a refrigeration apparatus 13 including a refrigerated means herein comprising an evaporator coil 14 of conventional construction.

For purposes of illustrating the invention, the mechanical structure of the refrigeration apparatus 13 is shown schematically only in the control circuit diagram. The invention herein resides in the control arrangement, and, thus, the mechanical structure of the refrigeration apparatus is shown only for environmental disclosure purposes.

The invention comprehends an improved control of the defrost operation in such a refrigeration apparatus 13. More specifically, the invention comprehends the provision of a defrost-initiation thermistor means 15 and a defrost-termination thermistor 16 connected in series from one power supply lead L As shown in the drawing, the thermistors 15 and 16 are closely associated with the evaporator coil 14 in the evaporator compartment 12. As shown, a trimming potentiometer 17 is connected between thermistor 15 and thermistor 16 and a second trimming potentiometer 18 is connected in series with the second thermistor 16. Also disposed within the evaporator compartment is a conventional evaporator fan 19 which is connected to one fixed terminal 20a of a single pole double throw door switch 20 having its movable contact 20b connected to power supply lead L The door switch 20 further includes a second fixed contact 20c which is connected through a suitable illuminating light 21 to the other power supply lead L As shown in the drawing, the door switch 20 is installed within the refrigerator compartment 22 of the refrigeration apparatus 13, as is light 21. Also disposed within the refrigerator compartment 22 is a third thermistor 23 and a trimming potentiometer 24 connected in series from power supply lead L A stile heater 25 and a mullion heater 26 for counteracting external cabinet sweating may be connected in parallel between power supply leads L and L in the conventional manner. An on-off switch 27 may be provided in power supply lead L Refrigeration apparatus 13 further includes a conventional compressor 28 and a conventional condenser fan 29 each of which is connected to power supply lead L The control 10 is completed by a control package generally designated 30 including a first relay 31, a second relay 32, a third relay 33, and a fourth relay 34. As shown, relay 31 comprises a conventional electromagnetic relay having a coil 31a and a normally open switch 31b having a fixed contact 310 and a moving contact 31d. Relay 32 comprises a thermal relay having a transducer, or electrical heater, 32a and a single pole, double throw switch 32b. Switch 32!) includes a moving contact 320 and a pair of fixed contacts 32d and 32a. Relay 33 comprises a conventional electromagnetic relay having a coil 33a and a normally open switch 33b having a fixed contact 33d and a movable contact 33c. Relay 34 comprises a thermal relay having a transducer, or heater, 34a and a normally closed switch 34b having a moving contact 34c and a fixed contact 34d.

' Trimming potentiometer 18 is connected through relay coil 31a to power supply lead L The fixed contact 310 of switch 31b of relay 31 is connected to power supply lead L and the moving contact 31d of switch 31b is connected to the heater 32a of relay 32. The heater in turn is connected to power supply lead L The moving contact 32c of switch 32b of relay 32 is connected to power supply lead L The fixed contact 32d of switch 32b is connected to the defrost heater 11 which in turn is connected to the power supply lead L The other fixed contact 32c of switch 321) is connected to the fixed contact 34d of switch 34b of relay 34, and the moving contact 34c of the switch 34b is connected to each of the evaporator fan 19, compressor 28, and condenser fan 29. The coil 33a of relay 33 is connected from trimming potentiometer 24 to the power supply lead L The moving contact 33c of switch 33b of relay 33 is connected to power supply lead L and the fixed contact 330. of switch 33b is connected to the heater 34a of the relay 34. Heater 34a in turn is connected to the power supply lead L to complete the electrical circuit of the control 10.

Thermistors 15, 16 and 23 comprise conventional posi tive temperature coefficient thermistors of conventional construction. Thermal relays 32 and 34 comprise conventional time delay thermal relays wherein the movable contacts 32c and 34c are actuated by bimetallic elements having a snap action so as to provide a toggle effect in the operation thereof. The trimming potentiometers 17 and 18 comprise conventional manually adjustable potentiometers.

The operation of the refrigeration apparatus 13, and in particular the operation of defrost control 10, is as follows. The thermistor 23 functions to maintain a relatively high resistance until the temperature of the refrigerator compartment 22 drops to the preselected low temperature. The resistor 24 is adjusted so that at the preselected temperature the resistance of the thermistor 23 will drop rapidly and permit a current flow through the relay coil 33a, thereby to energize the relay 33 and close the switch 331). As the heater 34a of relay 34 is in series with switch 3317, this will energize the heater 34a and after a period of time the heat from heater 34a will cause the switch 34b to open, thereby discontinuing further operation of the compressor 28, fan motor 29, and fan motor 19. When the temperature within the compartment 22 rises to above a preselected temperature the resistance of thermistor 23 increases, thereby causing the relay 33 to be de-energized and allow the switch 33b to open whereupon the relay 34 is de-energized and the compressor 28, fan motor 29, and fan motor 19 are again energized for refrigerating the apparatus.

The defrost control functions to control the defrosting of the evaporator coil 14 by suitably controlling the defrost heater 11. More specifically, the thermistor remains at a relatively high resistance until the evaporator coil 14 has a sufficient frost buildup thereon so as to cause a lowering of the temperature sensed by thermistor 15 to lower the resistance thereof to a preselected low value. The lowering of the temperature of the thermistor 15 may occur as a result of the continuous operation of the compressor resulting from the insulating effect of the frost build-up on the evaporator coil, or the thermistor may be in direct thermal contact with the frost on the evaporator coil as by a build-up thereof between the evaporator coil and the thermistor itself.

The thermistor 16 is associated with the evaporator coil 14 so as to sense the temperature of the evaporator coil at a point thereof which will indicate a thorough and complete defrosting of the coil. As shown herein, the thermistor 16 may be in direct thermal contact with the evaporator coil to obtain this functioning.

During normal operation of the evaporator coil 14, that is, when an insufficient amount of frost has built up thereon to initiate the defrositng cycle, the current flow from the power supply lead L through the series arrangement of thermistor 15, trimming potentiometer 17, thermistor 16, trimming potentiometer 18, and electromagnetic relay coil 31a to the power supply lead L is insuificient to energize the relay 31 to close switch 31b. The trimming potentiometers 17 and 18 may be set to effect this desired condition. At this time, the resistance of the thermistor 16 will be relatively low, but insufficiently low to permit the coil 31a to have suflicient current therein to operate the relay 31. However, when the thermistor 15 has the resistance thereof drop sufficiently as a result of the frost build-up on the evaporator coil 14, an increase in the current through the series circuit results, which is sufficient to energize the relay 31 to close switch 31b. The closing of switch 31b effects an energization of heater 32a of relay 32 as the switch 31b is connected in series therewith between the lines L and L After a short time, the switch 321) is thrown so that the moving contact thereof engages fixed contact 32d thereby connecting the defrost heater 11 from power supply lead L to the power supply lead L and initiating the defrost cycle. As indicated above, the switch 32b is a snap action switch and the moving contact 32c thereof remains closed with the contact 320! during the defrost operation.

The temperature of the evaporator 14 rises as a result of the heat energy delivered thereto from the defrost heater 11. However, the increase in the temperature sensed by the thermistor 15 is insufficient to increase the resistance thereof sutficiently to cause a drop-out of the relay 31. This results from the fact that the ampere-turns necessary to energize the relay are substantially greater than the ampere-turns necessary to maintain the relay energized. However, as the temperature sensed by the thermistor 16 rises at the completion of the defrosting operation, the resistance thereof increases rapidly to decrease the current flowing to the relay coil 31a to below its hold-in requirement, whereby the relay 31 is de-energized with the switch 31!) opening and thereby discontinuing energization of relay 32 which in turn discontinues further energization of the defrost heater 11. The switch 32b at this time returns to the normal position wherein the moving contact 320 engages the fixed contact 32e, once again establishing the circuit to the compressor 28 and fan motors 29 and 19 through switch 34b to permit the normal refrigeration cycle to recommence.

The trimming potentiometers 17 and 18 are normally adjusted at the time of assembly of the defrost control 10 and remain as set during the life of the apparatus. Thus, suitable fixed resistors may be utilized in lieu of the adjustable resistors, if desired.

Thus, defrost control 10 provides an improved accurate control of the defrost cycle in a refrigeration apparatus. Trimming potentiometer 24 is provided with a manual adjustment to effect a variable resistance for the combination of thermistor 23 and trimming potentiometer 24, thus providing a variable temperature at which relay 33 will function. The control 10 is extremely simple and economical of construction, while yet providing long trouble-free service.

While I have shown and described one embodiment of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as defined in the appended claims.

The embodiment of the invention in which an exclusive property or privilege is claimed is defined as follows:

1. In a refrigeration apparatus having refrigerated means and means for defrosting said refrigerated means, a control for selectively actuating said defrosting means, comprising:

manually adjustable first thermostat means in thermal transfer association with said refrigerated means for permitting a preselected electrical current flow in response to a preselected low temperature of said refrigerated means;

manually adjustable second thermostat means in thermal transfer association with said refrigerated means connected in series at all times with said first thermostat means for preventing said electrical current flow in response to a preselected high temperature of said refrigerated means; and

relay means having switch means electrically connected to said defrosting means, and coil means electrically connected in series with said first and second thermostat means for operating said switch means as a function of said current flow, said first and second thermostat means being constructed to permit said current flow to be suflicient to operate said relay means to energize said defrosting means when said first thermostat means senses a low temperature indicating a defrost requirement and to maintain said current flow sufficient to hold said relay means energized until said second thermostat means senses a high temperature indicating completion of a defrosting of said refrigerated means.

2. In a refrigeration apparatus having refrigerated means and means for defrosting said refrigerated means, a control for selectively actuating said defrosting means, comprising:

a first thermistor in thermal transfer association with said refrigerated means for sensing a requirement of defrosting thereof and having a first current control means connected in series therewith to vary electrical current flow therethrough;

a second thermistor in thermal transfer association with said refrigerated means for sensing a completion of a defrosting of said refrigerated means and a second current control means connected in series with the second thermistor to vary electrical current flow therethrough; and

relay means having switch means electrically connected to said defrosting means, and coil means electrically connected to said thermistors for operating said switch means as a function of the current flow through said current control means, said current control means being preselected to permit a suflicient current to flow to operate said relay means to energize said defrosting means when the first thermistor senses a defrost requirement and to maintain a sufficient current flow to hold said relay means energized until the second thermistor senses a completion of a defrosting of said refrigerated means.

3. The refrigeration apparatus of claim 2 wherein said current control means comprises manually adjustable resistor means in series with said thermistors for accurately calibrating the thermistor current control with the relay coil requirements.

4. The refrigeration apparatus of claim 2 wherein said current control means comprises a pair of manually adjustable resistors in series with said thermistors for accurately calibrating the thermistor current control with the relay coil requirements.

5. The refrigeration apparatus of claim 2 wherein said first thermistor is disposed to be directly contacted by frost on said refrigerated means for initiating the defrosting operation.

6. The refrigeration apparatus of claim 2 wherein said second thermistor is disposed to sense the temperature of the last portion of the refrigerated means to be defrosted.

7. In a refrigeration apparatus having refrigerated means and means for defrosting said refrigerated means, a control for selectively actuating said defrosting means, comprising:

a first thermistor in thermal transfer association with said refrigerated means for sensing a requirement of defrosting thereof and having current control means in series therewith to vary electrical current flow therethrough;

a second thermistor in thermal transfer association with said refrigerated means for sensing a completion of a defrosting of said refrigerated means and a second current control means connected in series with the second thermistor to vary electrical current flow therethrough; and

a pair of relays, a first one of said relays having (a) a coil connected in series with said current control means and (b) switch means, and the second of said relays having (c) a transducer connected in series with said switch means of the first relay and (d) switch means connected electrically in series with said defrosting means, said current control means being preselected to permit a suificient current to flow to operate said first relay thereby to energize said second relay and, in turn, said defrosting means when the sensing element of the first thermistor senses a defrost requirement and to maintain a sufiicient current flow to hold said first relay energized until the second thermistor senses a completion of a defrosing of said refrigerated means.

8. The refrigeration apparatus of claim 7 wherein said transducer comprises a heater and said switch means of the second relay comprises a thermally responsive switch.

References Cited UNITED STATES PATENTS 3,010,288 11/1961 Jacobs 62-156 X 3,039,278 6/1962 Thompson 62-156 X 3,222,882 12/1965 Sutton 62156 X MEYER PERLIN, Primary Examiner. 

1. IN A REFRIGERATION APPARATUS HAVING REFRIGERATED MEANS AND MEANS FOR DEFROSTING SAID REFRIGERATED MEANS, A CONTROL FOR SELECTIVELY ACTUATING SAID DEFROSTING MEANS, COMPRISING: MANUALLY ADJUSTABLE FIRST THERMOSTAT MEANS IN THERMAL TRANSFER ASSOCIATION WITH SAID REFRIGERATED MEANS FOR PERMITTING A PRESELECTED ELECTRICAL CURRENT FLOW IN RESPONSE TO A PRESELECTED LOW TEMPERATURE OF SAID REFRIGERATED MEANS; MANUALLY ADJUSTABLE FIRST THERMOSTAT MEANS IN THERMAL MAL TRANSFER ASSOCIATION WITH SAID REFRIGERATED MEANS CONNECTED IN SERIES AT ALL TIMES WITH SAID FIRST THERMOSTAT MEANS FOR PREVENTING SAID ELECTRICAL CURRENT FLOW IN RESPONSE TO A PRESELECTED HIGH TEMPERATURE OF SAID REFRIGERATED MEANS, AND RELAY MEANS HAVING SWITCH MEANS ELECTRICALLY CONNECTED TO SAID DEFROSTING MEANS, AND COIL MEANS ELECTRICALLY CONNECTED IN SERIES WITH SAID FIRST AND SECOND THERMOSTAT MEANS FOR OPERATING SAID SWITCH MEANS AS A FUNCTION OF SAID CURRENT FLOW, SAID FIRST AND SECOND THERMOSTAT MEANS BEING CONSTRUCTED TO PERMIT SAID CURRENT FLOW TO BE SUFFICIENT TO OPERATE SAID RELAY MEANS TO ENERGIZE SAID DEFROSTING MEANS WHEN SAID FIRST THERMOSTAT MEANS SENSES A LOW TEMPERATURE INDICATING A DEFROST REQUIREMENT AND TO MAINTAIN SAID CURRENT FLOW SUFFICIENT TO HOLD SAID RELAY MEANS ENERGIZED UNTIL SAID SECOND THERMOSTAT MEANS SENSES A HIGH TEMPERATURE INDICATING COMPLETION OF A DEFROSTING OF SAID REFRIGERATED MEANS. 